In installations that treat small to medium flow rates of water for distribution in the public drinking water mains network, it is particularly advantageous to treat the water by means of tubular membrane filters. Such installations must be capable of operating automatically and of providing filtered water in constant quantity and quality, regardless of variations in the quality of the source, and in spite of being made available to local authorities that do not have sufficient financial means to have qualified personnel in continuous attendance on such installations.
At present, three different filtering modes are known and in use with inside-skin tubular membranes:
(1) dead-end filtering where all of the water fed to the membranes passes through the membranes and is collected as a permeate (filtered water) from the periphery of the bundle of membranes. The flow rate at the axial outlet from the membranes is zero, and the membranes are cleaned periodically by backwashing whenever the thickness of the layer of matter in suspension as stopped by the membrane and as deposited thereon gives rise to a headloss which would increase pressure at constant flow rate to above an admissible threshold. The periodic backwashing has the purpose of removing this layer of deposited matter, and backwashing operations are triggered by automatic control means.
(2) cross-flow filtering, where only a portion of the water fed to the membranes passes through the membranes and is collected as a permeate, while the remainder is collected as a concentrate (i.e., non-filtered water) at the axial outlet from the membranes and is recirculated in a loop to the axial inlet of the membranes. Membrane cleaning is provided: firstly on a continuous basis by the water circulating over the membranes, thereby enabling a portion of the matter in suspension to be kept in suspension instead of being deposited on the membranes, and thus limiting cake thickness; and secondly by periodic backwashing.
(3) cross-flow filtering accompanied by continuous purging, using the above-described cross-flow filtering technique, where a portion of the unfiltered water including both the feed water and the concentrate is purged so as to limit the concentration of matter in suspension in the circuit.
Dead-end filtering is the most economic from the energy point of view since the flow rate of its single feed pump needs to be no greater than the flow rate of filtered water production. Cross-flow filtering requires an additional pump for recirculation; adding a continuous purge where water is returned to its source or to the drainage system is naturally even less economical.
In the event of the turbidity or the organic matter content of the feed water rising above the threshold at which good dead-end mode operation is possible, it must be possible to switch over to cross-flow mode filtering, and if the quality of the water becomes even worse, it will be necessary to be able to proceed with continuous purging. Conversely, if the quality of the water to be filtered is restored, then it is advantageous to be able to return to a more economical mode of operation.
At present, all three of these operating modes could be implemented in a single installation, with switching from dead-end mode to cross-flow mode being performed by switching on a recirculation pump, and with the addition of a purge being provided by opening an appropriate valve. The decision to change configuration would then be taken by the person responsible for supervision and as a function of the quality of the filtered water.
It would thus be advantageous to provide automatic switching from one mode to another without requiring human intervention.